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Homebrew EquipmentA place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Not too sure if this should be under 'Modern technology' section, but seems more relevant here. Mods feel free to move if required.

I have been thinking of getting one of these for a while now, especially as there is so little on AM these days, and I really want to listen to material that is age appropriate to my valve sets. Seeing as the price for the kit has now come down a bit, I decided to buy one.

It is more years than I care to remember since I built a kit, and thought this would be a challenge for my failing eyesight!

The kit duly arrived nicely packed with everything needed to plug and play, including a power supply.
After settling down with a cuppa, I read through the build instructions to familiarize myself with the kit, documentation is good, but could do with a bit more info in places for novices.

So, time to make a start. The single sided PCB is of decent quality and component idents are clear.
The first step is installing all the resistors. this is quite fiddly, as first you have to sort them for value, then bend the leads to the right spacing to fit the PCB.
Following the steps in the build document, all resistors were fitted and soldered in place.
As it was getting late, and did not want to make any mistakes, I decided to carry on the next day.

The next step was fitting the capacitors, dip switches, inductors and IC's.
As the IC's fit directly to the PCB, I utilised a small Bulldog clip as a heatsink while soldering in place. This did the trick in shunting the heat away from the devices.

After lunch, I fitted the rest of the components such as the five transistors, crystal, led, trimmer, voltage regulator and the various sockets.

With the PCB now finished, I screwed it in place in it's enclosure with both back and front panels fitted, and it was ready for testing.

I connected the audio input to my MP3 player, fitted the supplied aerial wire, checked that the dip switches were in their default settings and applied power.

I breathed a big sigh of relief as it worked first time
I used several radios when testing, and am very pleased with the results so far, the audio quality is very good once the input level is correctly set.
The range is adequate to cover my house and workshop, which is all I want from it.

All in all it was enjoyable to build if a little fiddly at times, but even with my less than perfect eyesight, very straightforward to construct, and a good saving over buying one ready built.

I have now completed the installation of the unit in it's permanent home, positioning of the supplied 9 foot wire aerial is critical for correct operation.
Trial and error has got it about right, however there is some issues with source material, if used with my laptop there is a background hum present regardless of output volume, this issue was sorted by earthing the unit to a nearby water pipe.
Using with any device with line level outputs requires attenuation to avoid overloading the unit, but once set correctly results are good, sounding at least equal in volume and quality to other AM stations.

I am using an MP3 player as the source, this is a great piece of kit capable of playing from both SD cards and USB memory sticks, it can also record to both from a line input.

I am also using a mini FM stereo tx as well, so I now am able to listen to my own material on my favourite radio, the Pye Fenman 2.

I have quite a few small capacity SD cards and memory sticks to fill with my own programme material, so this will range from the 1930's to the 1960's.
I have loaded up an SD card with original wartime broadcasts, it is really quite spooky listening to Winston Churchill on my 1937 HMV!

If the earthing is iffy, the Sputfure would work OK but the RF output would have to fight its way oot however it could. If this was via the power supply leads, there's the rectifier diodes in the way, which get turned on and off at mains frequency. So the radiated RF would vary at this rate.

Having an effective aerial and earth allows the RF to bypass this, so the hum disappears.

It is certainly working well now properly earthed, the manual does recommend that the unit is grounded.
I have been listening to it via my 1935 radiogram all morning, and there is no interference or drift.

I would certainly recommend it, I does for my radio collection what the Aurora has done for my 405 line TV's,
Mark

Most criticize the unit for poor Bass on transmission, this can be improved by changing C14 100uF to 470uF. I also found the tuning LED seemed to improve in indication by placing a 1N4148 diode across R34 (Anode to 0V and cathode to the cathode of D2).
I also drilled a suitable hole in the top cover for adjusting the antenna tuning as I found it needed re-tweaking with antenna placement.
I added some labels on the rear which should hopefully print to almost right dimensions by loading into MS Paint and setting the print size option to 100%.

The only odd thing I noticed about the design was the fact that they include a stabilised 12V DC supply with the unit yet the internal 3-terminal regulator is also 12V. I have not worked that one out yet

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Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

Its a nice kit, but has anyone else noticed that the coils supplied to build the atu section are or were NOT the values specified on the circuit diagram. I think this was done to stop people trying to connect wires over 9 foot, and getting an efficient match. I also understand this is only the situation with the 9k spaced UK kit the USA 10k kit has the correct values supplied

If the earthing is iffy, the Spitfire would work OK but the RF output would have to fight its way out however it could. If this was via the power supply leads, there's the rectifier diodes in the way, which get turned on and off at mains frequency. So the radiated RF would vary at this rate.

Without an earth at hand, just connect the negative (-) side of the power supply output to the electrical earth.
This is easily done in the case of a CB type power supply where the metal case is connected to earth, just put a link between negative and under one of the chassis screws.

The same goes for receiving - using a car radio indoors with a power supply, LW/MW will only become usable with the negative side grounded

FCC regulations mean it's easier to find off the shelf AM microtransmitters in the US than here in Britain. I use a Wild Planet Radio DJ toy transmitter, bought by a friend from a US branch of Toys'r'us. There's lots of info about this unit if you do some googling. I just had to fit a different crystal and tweak the tank circuit preset. It covers my house well without leaking too far.

Have just been given a Spitfire, trying it today I'd say the output power is compatible with the specification which says DC input to the RF amplifier is 100mW
(the weblink is now 6v6.co.uk/transmitters/spitfire-am-solid-state-transmitter-eu)
The input is shared with the series modulator device, and I'd estimate the RF output is in the region of 1dBm, so you don't really need to worry about being DF'ed out by the Radio Police.

A fault means it doesn't tune much below 600kHz, so I'll replace or increase the VCO capacitor. The VCO is the R-C oscillator in the 4046 which gives a phase-locked output to the untuned RF amplifier. I'll also do the suggested mods to the tuning indicator LED before I embark on a QRO enhancement

I made my Spitfire a bit more useful by housing it into a sloped front box.
A dual hex thumb switch allows easier frequency change and makes for a useful sig gen for aligning at the more common IF frequencies. I also added an optional phase shift oscillator and extended the tuning components together with a ON-ON-ON switch for easy adjustment of the output.

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Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

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